102 
PRESIDES r's ADDRESS. 
The water of a lake becomes salt if the outflow is cut off 
and evaporation is great : e.g., the Caspian Sea, at the mouth 
of the Volga, and Lake Shinva in East Africa. Fresh and 
salt water lakes may occur in the same drainage system, 
the lower lake always being the salt one : e.g., Lake Tiberias 
and the Dead Sea. 
In elevation, lakes range from 20.000 feet above sea-level 
(these are only small lakes) to 1272 feet below (the Dead Sea). 
The largest lakes at a great elevation are lake Chincay-Cocha, 
in Peru, at 13,000 feet, and Lake Titicaca at 12.500 feet. 
In si.:e they range from the 100,000 square miles of the Lakes 
of North America downwards ; and in depth down to 4000 
feet, as in Lake Baikal. 
They present, therefore, every kind of physical environment, 
and they bear the same kind of relation to the whole waters 
of the globe that islands do to the surface of the land. Hence 
the interest of the comparative study of the fauna and flora 
of fresh waters : for in the sea there are no sharply defined 
boundaries, and communication with all parts is easy. 
Each lake or lake system is a centre of differentiation, 
isolated and distinct, and with its own peculiar physical 
conditions. 
The water they contain may be of very various quality as 
regards temperature, density, chemical composition, and 
transparency. 
Temperature varies, not only with the altitude of the lake, 
and the latitude, but also with the change of season, and 
diurnally. It is more constant in deep than in shallow lakes, 
other factors being equal, and it is greatest at the end of the 
day and least at the end of the night. 
The temperature of the surface of expanses of water shows 
less variation than that of the layer of air immediately 
above it. 
Daily variations of temperature cease at a depth of 10 or 
